This Program Project represents a multidisciplinary collaborative effort designed to investigate and develop a definition and subtyping classification for dyslexia based on both a strong theoretical and empirical perspective and furthermore, to utilize sophisticated, state-of-the-art biological and genetic analytic approaches to investigate the validity of emerging subtypes. At the heart of this project is a strong theory-driven psychologic and linguistic assessment and analysis; an analysis that we believe is a prerequisite to the other modes of analysis. The approach we adopt here is guided by the hypothesis that language processes and abilities are distinct from other cognitive processes, and form a biologically-coherent system--in Fodor's terms, a "module" which is distinguished from other portions of the cognitive apparatus by special brain structures and by other anatomical specializations. The effort to sort out varieties of reading disorder based on cognitive-linguistic distinctions facilitates carrying out genetic-biologic analysis of any types of reading disorder that are identified by application of the test battery. Special genetic and biological studies carried out on cases of reading disability become a largely wasted effort unless the phenotypes have been accurately identified. Extensive experience has shown that the job of establishing the types of reading disorder cannot be accomplished simply by giving large numbers of standard tests to samples of affected children. Specifically, we intend to examine the manifestations of reading disorder within a large population of children who are learning disabled and cognitively impaired. The large and diverse population assessed with an in-depth test battery maximizes chances of uncovering different types of reading disability and utilization of biologic and genetic measures provide indices of validity. Further, assessments utilizing diverse strategies including early longitudinal and later epidemiological longitudinal samples and special populations insure that the emerging classification system will be meaningful and generalizable.